U.S. officials have confirmed that Americans are among the hostages being held following an attack by al Qaeda-linked insurgents on a BP joint-venture natural gas field in Algeria.

A senior intelligence official told ABC News that it currently appears three U.S. citizens are being held. However, the official cautioned that the number is subject to change as is the overall number of hostages.

The total number of hostages being held remains unclear, with reports ranging from 15 to more than 40. An official at BP told ABC News that there are approximately 700 local staff and contractors at the facility and 20 international BP staff and contractors at the In Amenas gas field in eastern central Algeria.

The attack was apparently carried out by a one-eyed jihadi leader, Mokhtar Belmokhar, who has been linked to a series of kidnappings of foreign nationals for ransom in North Africa that have earned tens of millions of dollars for al Qaeda's local affiliate, al Qaeda in the Islamic Maghreb (AQIM). Belmokhtar has been nicknamed Mr. Marlboro for his smuggling expertise.

In a briefing, State Department Spokesperson Victoria Nuland condemned the attacks and confirmed that U.S. citizens are among the hostages.

"In order to protect their safety," said Nuland, "I'm not going to get into numbers, I'm not going to get into names, I'm not going to get into any further details as we continue to work on this situation with Algerian authorities and also their employers."

"We are obviously closely monitoring the situation and in close contact with the government of Algeria and BP's security," said Nuland.

Two sources told ABC News that although public statements by the insurgents have linked the attack to French military operations in Mali, at least one foreign intelligence agency and the U.S. State Department believe the attack was too well organized and orchestrated to have been planned in that short a time frame. A senior State Department official also noted the attack was far from the Mali border.

Senior FBI officials in Washington and New York – the field office responsible for the region --are monitoring the Algerian hostage situation. They are seeing conflicting information on the number of U.S. hostages. If negotiations take place, the State Department would likely coordinate this through the host country, officials told ABC News.

The In Amenas gas field is jointly operated by BP, a Norwegian company and the Algerian national oil company and is 600 miles from Mali, where Al Qaeda-affiliated militants now control much of the country. Britons, Norwegians, and French and Japanese nationals are known to work at the facility and are said to be among the hostages. The British prime minister's office has confirmed that U.K. citizens are among those held, but did not disclose a number.

Al Qaeda linked militants called the Masked Brigade or the Khaled Abu al-Abbas Brigade claimed responsibility for the attack, saying it was retaliation for Algeria letting French jets use its airspace during its ongoing military operations in Mali. The news agency Agence France Presse (AFP) said it had received a telephone call from a person or persons who described themselves as hostage takers, and said, "We are members of al-Qaeda and we came from northern Mali. We belong to the[brigade] led by Mokhtar Belmokhtar."

Bemokhtar fought alongside the mujahideen in Afghanistan in the 1990s and lost an eye handling explosives. He has the reputation as an expert smuggler who prefers money to jihad. Over the past decade, he has allegedly mounted multiple kidnappings of foreign nationals in North Africa and extorted $70 million or more. In 2008, his group held two Canadian diplomats hostage for four months in the Sahara desert, taking them to Northern Mali and ultimately receiving a multi-million-dollar ransom for their release.

Last fall, he broke away from AQIM after other militants questioned his possible connection to weapons and drug smuggling. Belmokhtar, who is based in Northern Mali, remains affiliated with al Qaeda. He announced the formation of his new group in a video released on the web in December.

In a statement, BP confirmed that there was an armed attack on the In Amenas field early Wednesday morning.

"The site was attacked and occupied by a group of unidentified armed people at about 0500 U.K. time," said BP. "Contact with the site is extremely diffficult, but we understand that armed individuals are still occupying the In Amenas operations site."

Defense Secretary Leon Panetta said he didn't know whether there was any connection between the hostage-taking and the French military operation in Mali. "I do know that terrorists are terrorists," he said. "And terrorists take these kinds of actions not just in Algeria, they take them elsewhere."

"By all indications, this is a terrorist act and the United States strongly condemns these kinds of terrorist acts," said Panetta. "It is a very serious matter when Americans are taken hostage, along with others. ... I want to assure the American people that the United States will take all necessary and proper steps that are required to deal with this situation."